1. Field of Art
The present invention relates to a mode S secondary surveillance radar for detecting aircraft fitted with mode S transponders.
2. Description of Relevant Art
For air traffic control, ground stations have a mode S secondary surveillance radar (SSR mode S) adapted for detecting aircraft to prepare target reports to be used. The mode S secondary surveillance radar is adapted to acquire, from aircraft fitted with mode S transponders (mode S aircraft) in the radar coverage, their position data (slant ranges, azimuths), altitude data, aircraft IDs (identities), etc, prepare target reports thereon, and provide them to a control system to implement air traffic control. Such protocols are internationally unified (see e.g. ICAO Doc. 9684 “Manual of the Secondary Surveillance Radar (SSR) Systems”, “6.3 Surveillance Protocol”).
For example, as shown in FIG. 1, a mode S secondary surveillance radar 2 in the past includes an interrogation generator 22 that responds to requests from a channel management 21 by generating interrogations, of which signals are transmitted from a transmitter 23, through an antenna 24, to transponders on mode S aircraft. To interrogations thus sent out, replies from transponders are sent back, of which signals are received through the antenna 24 by a receiver 25, and input to a decoder 26 with listening windows opened in accordance with periods requested from channel management 11. The decoder 26 decodes input reply signals to acquire information including altitudes and IDs of targets, which is output to a report generator 27. Given information including aircraft IDs and altitudes, the report generator 27 prepares target reports containing information with aircraft IDs and altitudes inclusive, which are provided to air traffic control system.
The mode S secondary surveillance radar 2 acquires information of a respective aircraft by a protocol including: (1) an initial acquisition by a combination of all-call interrogations and replies thereto including replies from the aircraft; and (2) a tracking acquisition by a sequence of transactions each consisting of a selective interrogation and a reply to and from the aircraft as a target (see e.g. “Secondary Surveillance Rader SSR Mode S for Air Traffic Control” by Yoshio HASHIDA, Hisashi OHTOMO, & Yoshinori KUJI, Toshiba Review Vol. 59, No. 2, 2004, pp. 58-61).
As shown in FIG. 2, the mode S secondary surveillance radar 2 on the ground first performs an initial acquisition by all-call interrogations followed by replies from transponders on mode S aircraft (in a 1st scan and a 2nd scan), where it obtains data on aircraft to be targets, including their mode S addresses, positions, and the like. After this, the mode S secondary surveillance radar 2 starts tracking each aircraft as a target, entering an individual acquisition by sequential selective transactions (UF [uplink format]=4/DF [downlink format]=4 and UF=5/DF=5, in a 3rd scan), where it acquires data on altitude and aircraft ID of the target. Thereafter, the mode S secondary surveillance radar 2 keeps up a continual acquisition by selective transactions (UF=4/DF=4) to acquire data on altitude of the target, so long as the tracking is continued at a tracking processor 20 in FIG. 1 (in a 4th scan, a 5th scan, and unshown subsequent scans).
Each altitude acquiring transaction (UF=4/DF=4) provides a data on altitude, which is variable with time, as the aircraft flies in the air. Therefore, the mode S secondary surveillance radar 2 is adapted to acquire an altitude data every scan (each of the 3rd to 5th scans and subsequent scans) after the above-noted initial acquisition, and prepare a target report using an altitude data acquired in each scan.
The aircraft ID is a unique value (referred to as a “beacon code” or “mode A code”, as well) set to a flight plan defining, among others, a route and clock times of an associated flight, and once set up, it will not be altered during the flight as a rule. Therefore, the mode S secondary surveillance radar 2 executes an ID acquiring transaction (UF=5/DF=5) for acquiring an aircraft ID of the target in the 3rd scan just after the initial acquisition, and prepares target reports repeatedly using the aircraft ID acquired in the 3rd scan, without additional acquisition of aircraft ID in subsequent scans being the 4th, the 5th, et seq.
The aircraft ID may well be altered during a flight by a rare occurrence of an air traffic controller's instruction or in an emergency, where the DF=4 reply to be acquired every scan has its flight status (FS) changed. Therefore, detection of a changed flight status is coped with in an immediate sweep by execution of an additional ID acquiring transaction (UF=5/DF=5) for acquiring a new aircraft ID, before subsequent scans in which simply altitude acquiring transactions (UF=4/DF=4) are to be executed.
For each scan, which is limited in time, the mode S secondary surveillance radar 2 repeatedly employs an aircraft ID once acquired, allowing for an efficient use of the limited scan time. That is, the mode S secondary surveillance radar 2 is afforded to devote a remaining scan time to other aircraft, for transactions and data communications therewith.
Such a repetition in use of an aircraft ID once acquired is supported by a CRC function in the mode S reply that implements a reliable data acquisition. That is, the mode S secondary surveillance radar 2 relies on the aircraft ID once acquired, to repeat the use.
However, the CRC function in mode S reply has a nominal error rate of 10−7, with an anxiety of a failure to detect an occurrence of error with a probability of 10−7. Should an error occur in a transaction (UF=5/DF=5) in an immediate scan after the initial acquisition, target reports containing an erroneous aircraft ID might be prepared in subsequent scans, for use in air traffic control, which is problematic to the safety of air traffic control.
The present invention has been devised with this point in view. It therefore is an object of the present invention to provide a mode S secondary surveillance radar adapted for acquisition of a correct aircraft ID to output a reliable target report with contribution to an ensured safety of air traffic control.